1. Field of the Invention
This invention relates to methods and apparatuses for liquefying natural gas. In another aspect, the invention concerns a liquefied natural gas (LNG) facility employing a cooling system operable to enhance the performance of one or more gas turbines used in the LNG facility.
2. Description of Related Art
Cryogenic liquefaction is commonly used to convert natural gas into a more convenient form for transportation and/or storage. Because liquefying natural gas greatly reduces its specific volume, large quantities of natural gas can be economically transported and/or stored in liquefied form.
Transporting natural gas in its liquefied form can effectively link a natural gas source with a distant market when the source and market are not connected by a pipeline. This situation commonly arises when the source of natural gas and the market for the natural gas are separated by large bodies of water. In such cases, liquefied natural gas (LNG) can be transported from the source to the market using specially designed ocean-going LNG tankers.
Storing natural gas in its liquefied form can help balance periodic fluctuations in natural gas supply and demand. In particular, LNG can be “stockpiled” for use when natural gas demand is low and/or supply is high. As a result, future demand peaks can be met with LNG from storage, which can be vaporized as demand requires.
Several methods exist for liquefying natural gas. Some methods produce a pressurized LNG (PLNG) product that is useful, but requires expensive pressure-containing vessels for storage and transportation. Other methods produce an LNG product having a pressure at or near atmospheric pressure. In general, these non-pressurized LNG production methods involve cooling a natural gas stream via indirect heat exchange with one or more refrigerants and then expanding the cooled natural gas stream to near atmospheric pressure. In addition, most LNG facilities employ one or more systems to remove contaminants (e.g., water, acid gases, nitrogen, and ethane and heavier components) from the natural gas stream at different points during the liquefaction process.
Typically, LNG facilities employ one or larger, multi-stage refrigerant compressors to circulate refrigerant used to cool the natural gas feed stream processed in the facility. These industrial compressors are often driven by gas turbines, which combust fuel with a stream of compressed air to generate power that can then be utilized to drive the compressor. The performance of these gas turbines, which can generally be correlated to the turbine's power output and efficiency, is highly dependent on ambient air conditions. For example, increases in ambient air temperature typically lead to decline in turbine power output. Reductions in turbine power output translate to diminished compressor performance, which, ultimately translates to reduced LNG production. Thus, it may be desirable to maintain and/or increase LNG production through enhanced gas turbine operation.